1. Field of the Invention
This invention relates to method and apparatus for supporting a rock formation above an underground excavation and, more particularly, to a flexible multi-strand steel cable anchored in a bore hole of the rock formation to reinforce the strata of the rock formation.
2. Description of the Prior Art
It is well known to reinforce and stabilize underground rock formations, such as a coal mine roof, a subway tunnel or similar subterranean structure or to strengthen a rock mass by the use of anchor bolts inserted within bore holes drilled in the rock formation. U.S. Pat. No. 5,244,314 discloses the type of anchor bolt that utilizes a reinforcing rod of a preselected length, for example 6 to 10 feet, that is inserted in a hole drilled into the rock formation. The end of the bolt is anchored in the bore hole by either engagement of a mechanical expansion shell with the wall of the surrounding rock formation or chemically anchoring the bolt by a multi-component resin system or cement grout to the surrounding rock formation. The bolt can be anchored in the bore hole using resin or grout alone in a passive system where the bolt is not placed in tension. A bolt which is mechanically anchored in the bore hole is placed in tension, and a tensioned roof bolt can also be chemically bonded to the rock formation.
Under specific rock formation conditions it is known to use lengths of multi-strand high strength steel cables anchored in bore holes to reinforce an unstable rock formation. As with a steel bolt, a cable bolt can be both chemically and mechanically anchored in the bore hole. A cable bolt can be anchored using an expansion shell assembly, as disclosed in U.S. Pat. No. 5,244,314, or with a resin bonding system. Other types of mechanical anchors can also be used in combination with a resin bonding system to tension an anchored cable bolt.
Because of the flexible nature of a cable bolt it is particularly useful in the reinforcement of rock strata that is subject to significant horizontal shifting of the strata layers. This shifting movement generates shear forces which can break a steel bar bolt. However, a cable bolt can withstand substantial lateral deflection due to shifting of the rock strata before it breaks.
The flexibility of cable bolts is also particularly adaptable for insertion in bore holes of a considerable length, particularly where the length of the bore hole exceeds the height of the mine roof. Where steel bar bolts are installed in bore holes that exceed the height of the passageway beneath the structure to be supported, short lengths of bars must be coupled together. Each length is individually coupled to the preceding bar. The coupled bars are advanced sequentially in the bore hole. This is a time consuming and expensive task. On the other hand, cable bolts are flexible and can be bent as they are inserted in the bore holes. Thus, for example, in an underground passage having a roof height of 6 to 8 feet a continuous length of cable bolt can be efficiently inserted into a 60 foot bore hole above the passageway without coupling together sections of the bolt. A bolt of this length can be mechanically or chemically anchored in the bore hole or a combination of both systems used.
The effectiveness of a cable bolt to support a rock formation is determined to a great extent by the capacity of the cable bolt to resist pull-out under the loads exerted by fractured rock strata. A number of devices have been proposed for anchoring cable bolts in bore holes, such as steel buttons and birdcages, as described in a report published by the United States Department of the Interior, Bureau of Mines entitled "Laboratory Evaluation of Cable Bolt Supports" (in two parts) by J. M. Goris, published 1991.
In a system of anchoring a cable bolt using cement grout, buttons and birdcages increase the resistance of the cable bolt to pull-out by compressing the grout into contact with the wall of the bore hole. This increases the engagement or bonding of the grout with the surrounding rock strata. With the cable bolt adhered to the grout, the cable bolt is securely anchored within the bore hole to the surrounding rock formation.
Because of the advantages provided by the flexible nature of cable bolts to withstand shear forces generated by lateral movement of rock strata, efforts have been made to provide the cable bolt with features that facilitate rapid installation of a tensioned cable bolt. To accommodate the upward insertion of the cable in a bore hole and rotation of the bolt to effect mixing of a two component resin system, a mechanical drive head is installed on the end of the cable that extends out of the bore hole. One example of a drive head on the end of a cable bolt is disclosed in U.S. Pat. No. 4,798,501 where a roof bolting machine engages the drive head to advance it upwardly into the bore hole and rotate the entire cable bolt to effect mixing of a two component resin system. It is essential that the drive head be nonrotatably connected to the end of the cable so that the rotation is transmitted from the drive head to the cable. If the drive head is not secured to the cable, the drive head will rotate on the cable and the rotation will not be transmitted to the cable.
The combination of a gripping wedge and a drive collar with a tapered bore for preventing the drive collar from moving on the end of the cable is disclosed in a paper entitled "Cable Bolting" by G. Daws published in the February 1991 edition of The Mining Engineer. At the emergent end of the cable bolt a pair of wedges having tapered exterior surfaces is received within a cylindrical collar having a bore tapered in the opposite direction of the taper on the wedges. When the collar is advanced into surrounding relation with the wedges, the wedges are compressed into gripping engagement with the cable. The collar is frictionally engaged by the mating tapered surfaces to the wedges. Rotation transmitted to the surrounding collar rotates the entire cable to facilitate longitudinal insertion and rotation of the cable in the bore hole. A similar arrangement of an internally tapered drive collar engaging tapered wedges gripping a cable bolt is disclosed in U.S. Pat. Nos. 5,230,589 and 5,259,703.
The provision of a drive collar having a tapered bore surrounding tapered wedges on the end of a cable permits installation of a cable bolt by the same machinery used to install rebar bolts. The cable bolt can be rapidly advanced regardless of its length into the bore hole and then rotated to effect mixing of resin components and/or set an expansion shell assembly in the bore hole.
With known devices, it is the conventional practice to transmit rotation to the surrounding drive collar which is advanced on the wedges to the point where the wedges are compressed into gripping engagement with the cable. The drive collar is frictionally engaged by the mating tapered surfaces to the wedges. If the drive collar is not frictionally engaged to the wedges, then the collar can rotate or slip on the wedges and rotation is not transmitted through the wedges to the cable. If the cable does not rotate an expansion anchor can not be set or the components of a resin system mixed.
While it is known to use a combination collar and wedge set to transmit rotation to the end of a cable bolt, there is need for method and apparatus for securing a drive head to a cable bolt that eliminates the problem of slippage of the drive head on the end of the cable bolt.